Cleaning of metallic contaminants from the surface of polycrystalline silicon with a halogen gas or plasma

Abstract

A process for removing metallic contaminants from the surface of chunk or granular polycrystalline silicon is disclosed. The metallic contaminants are exposed to a gas-phase halogen etchant which reacts with the metallic contaminants to form metal halides. The gas-phase halogen etchant is preferably a halogen plasma. The metal halides are volatilized into an atmosphere adjacent to the surface of the polycrystalline silicon, and purged therefrom. The cleaned polycrystalline silicon is used to prepare a mass of molten silicon for use in producing a single crystal silicon ingot by the Czochralski method.\!

We claim: 
     
       1. A process for removing metallic contaminants from the surface of chunk or granular polycrystalline silicon comprising: a. exposing the metallic contaminants and the polycrystalline silicon to a gas-phase halogen etchant in a reaction chamber, the halogen etchant reacting with the exposed metallic contaminants to form metal halides;   b. volatilizing the metal halides of at least some metallic contaminants into an atmosphere adjacent to the surface of the polycrystalline silicon;   c. purging the volatilized metal halides from the atmosphere; and   d. volatilizing at least some of the metal halides not volatilized in the reaction chamber in a crystal puller during a subsequent crystal pulling step.   
     
     
       2. The process as set forth in claim 1 wherein the gas-phase halogen etchant comprises a halogen plasma. 
     
     
       3. The process as set forth in claim 2 wherein the halogen plasma is a low pressure halogen plasma. 
     
     
       4. The process as set forth in claim 2 wherein the halogen plasma is generated by energizing a plasma source-gas comprising a halogen gas or a halogen-containing compound gas in a microwave field having a frequency ranging from about 1.0 GHz to about 4.0 GHz. 
     
     
       5. The process as set forth in claim 2 wherein the metallic contaminants include Na, K, Ca, Ti, Cr, Fe, Co, Ni or Cu, and wherein the halogen plasma is generated by energizing a source-gas comprising a halogen gas or a halogen-containing compound gas at a pressure ranging from about 1 Pa to about 1.3×10 4  Pa. 
     
     
       6. The process as set forth in claim 2 wherein the halogen plasma is generated by energizing a source-gas comprising a gas selected from the group consisting of fluorine, chlorine, bromine, iodine, chlorocarbons, fluorocarbons, chloroflourocarbons, and halogenated hydrocarbons. 
     
     
       7. The process as set forth in claim 1 wherein the gas-phase halogen etchant comprises a halogen gas or a halogen-containing compound gas. 
     
     
       8. The process as set forth in claim 7 wherein the halogen gas or halogen-containing compound gas is at a temperature sufficient to react with metallic contaminants to form metal halides. 
     
     
       9. The process as set forth in claim 7 wherein the metallic contaminants include Na, K, Ca, Ti, Cr, Fe, Co, Ni or Cu, and wherein the temperature of the halogen gas or halogen-containing compound gas ranges from about 25° C. to about 900° C. 
     
     
       10. The process as set forth in claim 7 wherein the gas-phase halogen etchant is a gas selected from the group consisting of fluorine, chlorine, bromine, iodine, chlorocarbons, fluorocarbons, chloroflourocarbons, halogenated hydrocarbons, and boron trichloride. 
     
     
       11. The process as set forth in claim 1 wherein the metal halides volatilized in the reaction chamber include the halides of Na, K, Fe, Ni, or Cu, wherein the temperature of the surface of the polycrystalline silicon in the reaction chamber ranges from about 200 ° C. to about 500° C. and wherein the pressure of the reaction chamber is less than about 130 Pa. 
     
     
       12. The process as set forth in claim 1 wherein the surface of the polycrystalline silicon is etched by the gas-phase halogen etchant at least until the concentration of metallic contaminants at the etched surface is about equal to the concentration of metallic contaminants within the bulk of the polycrystalline silicon. 
     
     
       13. The process as set forth in claim 1 wherein the thickness of the polycrystalline silicon etched by the gas-phase halogen etchant ranges from about 1 μm to about 50 μm. 
     
     
       14. A process for removing metallic impurities from the surface of chunk or granular polycrystalline silicon, the process comprising: a. exposing the metallic contaminants and the polycrystalline silicon to a low pressure halogen plasma which reacts with the exposed metallic contaminants to form metal halides and etches the polycrystalline silicon, the halogen plasma being generated in a reaction chamber by energizing a source-gas comprising a halogen gas or a halogen-containing compound gas in a microwave field having a frequency ranging from about 1.0 GHz to about 4.0 GHz, the polycrystalline silicon being etched by the halogen etchant until the bulk polycrystalline silicon is reached;   b. volatilizing the metal halides into an atmosphere adjacent to the surface of the polycrystalline silicon, the halides of at least some metallic contaminants being volatilized in the reaction chamber, the metal halides not volatilized in the reaction chamber being volatilized in a crystal puller during a subsequent crystal pulling step, wherein the temperature of the surface of the polycrystalline silicon is greater than about 250° C. and the pressure of the reaction chamber is less than about 1300 Pa; and   c. purging the volatilized metal halides from the atmosphere.   
     
     
       15. The process as set forth in claim 14 further comprising the step of purging the reaction chamber with an inert gas. 
     
     
       16. A process for removing metallic contaminants from the surface of chunk or granular polycrystalline silicon prior to use thereof for production of single crystal silicon, the process comprising: a. exposing the metallic contaminants and the polycrystalline silicon to a halogen plasma which reacts with the exposed metallic contaminants to form metal halides;   b. volatilizing the metal halides into an atmosphere adjacent to the surface of the polycrystalline silicon; and   c. purging the volatilized metal halides from the atmosphere.   
     
     
       17. The process as set forth in claim 16 wherein the halogen plasma is generated by energizing a plasma source-gas comprising a halogen gas or a halogen-containing compound gas in a microwave field having a frequency ranging from about 1.0 GHz to about 4.0 GHz. 
     
     
       18. The process as set forth in claim 16 wherein the halogen plasma is generated by energizing a plasma source-gas comprising a halogen gas or a halogen-containing compound gas, the plasma source-gas being at a temperature ranging from about 25° C. to about 300° C. 
     
     
       19. The process as set forth in claim 16 wherein the temperature of the surface of the polycrystalline silicon in the reaction chamber ranges from about 200° C. to about 500° C. and wherein the pressure of the reaction chamber is less than about 130 Pa. 
     
     
       20. The process as set forth in claim 16 wherein the halogen plasma is a low pressure halogen plasma. 
     
     
       21. The process as set forth in claim 16 wherein the halogen plasma is generated by energizing a plasma source-gas comprising a halogen gas or a halogen-containing compound gas at a pressure ranging from about 1 Pa to about 1.3×10 4  Pa. 
     
     
       22. The process as set forth in claim 16 wherein the halogen plasma is generated by energizing a plasma source-gas comprising a gas selected from the group consisting of fluorine, chlorine, bromine, iodine, chlorocarbons, fluorocarbons, chlorofluorocarbons, and halogenated hydrocarbons. 
     
     
       23. The process as set forth in claim 16 wherein the metallic contaminants include Na, K, Ca, Ti, Cr, Fe, Co, Ni or Cu, and wherein the halogen plasma is generated by energizing a source-gas comprising a halogen gas or a halogen-containing compound gas at a pressure ranging from about 1 Pa to about 1.3×10 4  Pa. 
     
     
       24. The process as set forth in claim 16 wherein the surface of the polycrystalline silicon is etched by the halogen plasma at least until the concentration of metallic contaminants at the etched surface is about equal to the concentration of metallic contaminants within the bulk of the polycrystalline silicon. 
     
     
       25. The process as set forth in claim 16 wherein the surface of the polycrystalline silicon is etched by the halogen plasma at least until the thickness of polycrystalline silicon removed therefrom ranges from about 1 μm to about 50 μm. 
     
     
       26. The process as set forth in claim 16 wherein the metallic contaminants and the polycrystalline silicon are exposed to the halogen plasma in a reaction chamber, and wherein the metal halides are volatilized in the reaction chamber. 
     
     
       27. The process as set forth in claim 26 wherein the temperature of the surface of the polycrystalline silicon is sufficient, at the pressure of the reaction chamber, to volatilize the metal halides. 
     
     
       28. The process as set forth in claim 26 wherein the metallic contaminants include Na, K, Ca, Ti, Cr, Fe, Co, Ni or Cu; wherein the temperature of the surface of the polycrystalline silicon in the reaction chamber is greater than about 250° C.; and wherein the pressure is less than about 1300 Pa. 
     
     
       29. The process as set forth in claim 16 wherein the metallic contaminants and the polycrystalline silicon are exposed to the halogen plasma in a reaction chamber, the metal halides of at least some metallic contaminants are volatilized in the reaction chamber and the metal halides not volatilized in the reaction chamber are volatilized in a crystal puller during a subsequent crystal pulling step. 
     
     
       30. A process for preparing a mass of molten silicon from polycrystalline silicon for use in producing a single crystal silicon ingot by the Czochralski method, the process comprising: a. removing metallic contaminants from the surface of chunk or granular polycrystalline silicon by exposing the metallic contaminants to a halogen plasma which reacts with the exposed metallic contaminants to form metal halides; volatilizing the metal halides into an atmosphere adjacent to the surface of the polycrystalline silicon; and purging the volatilized metal halides from the atmosphere;   b. charging the cleaned polycrystalline silicon to a crucible; and   c. melting the polycrystalline silicon.